The blood flow in the human heart is very intricate, and it is presumed that the fluid dynamics in the heart are greatly influenced by even minor changes of the cardiovascular function. For example, vortices in blood flow contribute to the dynamic equilibrium between the hyperelastic heart tissue and the intraventricular blood pressure and the shear stress. The vortices are likely to be critical to the energetic properties of the blood flow. Heart function and quality of the intraventricular fluid dynamics are presumed to be closely related. Dilatation or dyskinesia could result in distortion of vortex, local stagnation and reduced flow exchange accompanied by higher risk of thrombus formation. Changes in fluid dynamics, e.g. changes of vortex or eddies as well as partially reversed blood flow movement are likely to be an early indicator of changes of the heart function, even before they become manifest in the heart muscle itself.
Possibly, more specific analysis of the blood flow may hence provide essential information concerning the heart function. Nevertheless, cardiovascular fluid dynamics are rarely utilized in everyday clinical practice, especially since technical means for the examination thereof are missing. Hence, there is a demand for new techniques for the examination of fluid dynamics of the heart function.
Medical imaging methods for displaying large blood vessels or heart chambers are for example Phase Contrast Cardiac Magnetic Resonance (CMR), by which a three dimensional (3D) velocity vector field may be achieved. However, this method does not provide optimal space resolution nor acquisition frequency, and the results must be averaged over a large number of heartbeats, so that minor fluctuations e.g. in vortex/eddy formation, are not detected.
An alternative is the Ultra Sound Color Doppler, which however shows the basic limitation that solely the component of the blood flow parallel to the ultra sound beam is detected. In order to overcome this limitation, methods have been developed for reconstructing a velocity vector field from the color Doppler velocity information (refer to US 2012/0265075 A1).
A more versatile echocardiographic technique for imaging and evaluating the blood flow in the large vessels or in the heart, respectively, in real time is known under the name of Echo-PIV (Echo particle image velocimetry). This technique is for example described in the articles of P. P. Sengupta and G. Pedrizzetti et al. “Emerging Trends in CV Flow Visualization”, J. Am. Coll. Cardiol. Img. 2012; Vol. 5; No. 3; pp. 305-316, as well as D. R. Munoz, M. Markl et al. “Intracardiac flow visualization: current status and future directions”, European Heart Journal—Cardiovascular Imaging, 1 Aug. 2013, doi:10.1093/ehjci/jet086. The contents of these documents are enclosed in this application.
The Echo-PIV technique is based on that a time series of 2D (two dimensional) or 3D (three dimensional) ultra sound images of the heart is recorded. Generally, acquisition over one single heartbeat is sufficient, since the time resolution is very high. During this procedure the patient may have also been given a suitable ultra sound contrast agent. In post-processing, the individual particles visible in the blood (e.g. reflecting particles of contrast agent) are traced from image to image. In doing so, velocity vector fields may be calculated and visualized, such as e.g. represented in FIG. 1. This may be done both in 2D and 3D. In 2D ultra sound images solely the blood flow in the image plane is detected. By “ultra sound image” in this application especially a B mode image is understood, if not stated otherwise.